(From the application abstract) Primary carnitine deficiency is an autosomal recessive disorder caused by defective carnitine transport. Carnitine is essential for fatty acid oxidation, and its deficiency results in hypoketotic hypoglycemia, skeletal and heart myopathy which are preventable by dietary carnitine. The Organic Cation Transporter with Nucleoside binding site (OCTN2) is a high-affinity carnitine transporter, which was cloned based on its homology with OCTN1 (which does not transport carnitine). Its role in carnitine deficiency was confirmed by the identification of nonsense mutations in OCTN2 in patients with early presentation of primary carnitine deficiency. The principal investigator proposes to test the hypothesis that families with primary carnitine deficiency have a spectrum of mutations in the organic cation transporter OCTN2, and that the degree of functional impairment of the transporter caused by these mutations correlates with the severity of the clinical presentation. To test this hypothesis, the following specific aims will be pursued: 1) Identification of mutations in the OCTN2 gene in families with primary carnitine deficiency. 2) Expression of missense mutations identified in these patients in Chinese Hamster Ovary (CHO) cells to confirm their causative role and to characterize their effect on carnitine transport. A correlation will be established between phenotype of the proband and residual carnitine transporter activity of the relative mutant. 3) Definition of domains of OCTN2 involved in carnitine recognition and transfer by evaluating carnitine transport in chimeric transporters, created by swapping domains between homologous portions of OCTNI and OCTN2. 4) Construction of site-directed mutations in critical domains of the OCTN2 carnitine transporter and determination of their effect on carnitine transport. This research will characterize mutations responsible for primary carnitine deficiency and clarify the function of a new class of membrane transporters whose alteration may cause other types of human diseases.